1. Field of the Invention
The present invention relates to a novel styrene type resin and its production method. More specifically it relates to a styrene type resin having excellent strength which is characterized in having a yield point and a break point in a tensile stress-strain test. These characteristics are due to the resin comprising a specific proportions of styrene type structural units and structural units having a long alkyl chain defined by specific structural formula and due to the restriction of the total amount of styrene type monomer, styrene type dimer, styrene type trimer and polymerization solvent.
Further, this invention relates to a method of producing the above-mentioned resin in high productivity.
2. Description of the Prior Art
Styrene type resins have heretofore been used extensively as plastic materials for household articles, electric appliances, etc., due to excellence in transparency, moldability and modulus. Due to the recent price hike in raw materials, resins which are relatively low in cost are replacing high quality resins and demand is growing. Herewith improvement of styrene type resins in strength and moldability to diversify the usage and to enhance the productivity are now in public demand.
An increase of the average molecular weight is a publicly known method to obtain a styrene type resin having high strength. However, the increase of the average molecular weight deteriorates the moldability of the resin.
Although using a plasticizer is a well-known method to supplement moldability, the addition of a plasticizer deteriorates heat resistance, modulus and strength. There are some fields which require high speed molding to enhance the productivity. The improvement in strength and moldability by means of enlarging the average molecular weight and using a plasticizer alone can not respond to these fields.
Whereas in some fields, resins, e.g., for large-sized shaped article use, require strength and moldability considerably more than heat resistance and stiffness. While it is common practice to use a plasticizer, adding a plasticizer to a conventional styrene type resin causes deterioration in strength. Therefore, the practice does not necessarily satisfy the market demand. Since the addition of a plasticizer deteriorates the strength of a resin, a method to produce styrene type resins having strength which is sufficient to fulfill the market demand and having excellent moldability as well must be sought.
Many shaped articles produced by foaming styrene type resins are used for wrapping food. A styrene type resin having wide molding width at heat molding, namely, a styrene type resin having good elongation is in demand.
Japanese Patent Publication No. 61231/1987 discloses that minimizing the content of low molecular weight polymer components is efficient in improving the strength of styrene type resins. Further, Japanese Patent Applications Laid-open Nos. 8304/1985 and 13805/1985 disclose a method of producing a styrene type resin excellent in strength according to which the resin contains a high molecular weight polymer component which is produced by using a multifunctional organic peroxide of decomposition type at low temperature. It can be expected that the strength of the resin is improved by these methods to some extent, but it can not be expected that the brittle breaking pattern is changed.
In other words, in a tensile stress-strain test, the elongation is in direct proportion to the tensile stress until break and there is no yield point. The increase of molecular weight does not change this feature. For example, the area below the tensile stress-strain curve, which is a strength standard, changes only by few percents.
Even if a multifunctional organic peroxide of the low temperature decomposition type is employed, remarkable improvement of a styrene type resin in strength can not be expected unless a proper styrene type resin is produced by a proper method.
Further, polymerization temperature and final polymerization rate are low in the methods of Japanese Patent Application Laid-open Nos. 8304/1985 and 13805/1985. Therefore, a large quantity of styrene type dimer and styrene type trimer are produced because unreacted styrene type monomers are exposed to a high temperature at the recovery system. Further, a low molecular polymer with molecular weight of no more than 20,000 is produced in large quantity. As a result, breakage occurs without having a yield point in a tensile stress-strain test and strength improvement is not sufficiently high.
Japanese Patent Application Laid-open No. 1525175/1985 discloses the use of multifunctional organic peroxide of the decomposition type at low temperature in the copolymerization reaction of styrene and .alpha.-methyl styrene. However, expected improvement does not occur due to the employment of the polymerization condition of the above-mentioned official document. As stated so far, conventional styrene type resins have been improved in many ways, however, there is still no satisfactory change in the brittleness characteristic of these resins.
It is publicly known that the addition of an elastomer changes the pattern. It is a so-called impact resistant polystyrene. Transparency is one of the primary characteristics of styrene type resin, however this addition causes the resin to be opaque.
Further, it is publicly known that the addition of a copolymerizable second monomer to the styrene type monomer improves the strength and flow property. For example, AS resin is produced by reacting styrene with acrylonitrile and BAS resin is produced by reacting styrene with butyl acrylate. The first objects are accomplished for these styrene type resins by the addition, however, some of the characteristics of the resins are lost. AS resin is remarkably deteriorated in flow property and is worsened in color tone and BAS resin is remarkably deteriorated in heat resistance.
Copolymerized resins containing a second monomer have the defect of losing transparency while mixing with styrene type resin. Therefore, these resins can not be used under the same conditions as styrene type resins.